In any transition series, from left to right, the d-orbitals are progressively filled and their properties vary accordingly.
Q. Which element do you expect to have the highest melting point?

Options

A. La
B. W
C. Pt
D. Os

Explanation

To determine which element has the highest melting point among the transition series elements mentioned (Lanthanum, Tungsten, Platinum, and Osmium), we will analyze their electronic configurations, oxidation states, and bonding characteristics.

### Step-by-Step Solution:

1. **Identify the Elements and Their Electronic Configurations**:
- Lanthanum (La): Atomic number 57, electronic configuration: [Xe] 5d¹ 6s²
- Tungsten (W): Atomic number 74, electronic configuration: [Xe] 5d⁴ 6s²
- Platinum (Pt): Atomic number 78, electronic configuration: [Xe] 5d⁹ 6s¹
- Osmium (Os): Atomic number 76, electronic configuration: [Xe] 5d⁶ 6s²

2. **Determine the Oxidation States**:
- Lanthanum can exhibit oxidation states of +2 and +3.
- Tungsten can exhibit oxidation states from +2 to +6.
- Platinum typically exhibits a +1 oxidation state but can also show +2.
- Osmium can exhibit oxidation states of +2, +3, and +4.

3. **Analyze the Bonding Characteristics**:
- Tungsten, with its ability to exhibit a +6 oxidation state, can form six bonds with other atoms. This high coordination number leads to stronger metallic bonding due to the increased number of interactions between atoms.
- The strong metallic bonds in tungsten contribute significantly to its high melting point.

4. **Compare the Melting Points**:
- Tungsten is known for having the highest melting point of all elements, which is approximately 3422 °C.
- Lanthanum, Platinum, and Osmium have lower melting points compared to Tungsten.

5. **Conclusion**:
- Based on the analysis of oxidation states and bonding characteristics, **Tungsten (W)** is expected to have the highest melting point among the elements listed.

### Final Answer:
Tungsten (W) has the highest melting point among the elements Lanthanum, Tungsten, Platinum, and Osmium.